1. Field of the invention
The present invention relates to systems and methods for controlling RF output power.
2. Discussion of the Background
To address a variety of applications, RF power supplies (e.g., RF power supplies used in Induction heating) must employ a method of varying RF output power. There are many ways to accomplish this. Some architectures simply supply a variable DC source to the RF output power stage of the power supply. The result has the advantage of a continuous (rather than discontinuous) RF output with a variable and easily measurable amplitude whose heating effects and harmonics are predictable. Disadvantages of this method are that it is costly, complex and generally requires twice the silicon (in power semiconductors) than is required by a direct off-line architecture.
Varying (controlling) the RF power level using a direct off-line architecture is less costly, however this approach presents a different set of challenges to the designer. Because the DC feed to the RF output power stage (or “final power stage”) cannot be directly controlled (having been simply derived directly from the AC line), another method of RF output regulation must be found. Commonly, Pulse Width Modulation (PWM) is chosen as means to accomplish this.
In PWM schemes, the final power stage is allowed to conduct during a controlled portion of each full RF cycle. Ignoring dead time between each half of the switching cycle, full power is generated when the output switching devices of the final power stage are allowed to conduct 100% of their allotted “on time,” while lowering the power output is done by reducing the amount of time that the output switching devices are allowed to conduct. At 0% power they conduct 0% of the time.
If the RF frequency does not change, power can be controlled by simply allocating an absolute amount of time that the switching devices are allowed to conduct, rather than a proportional value.
In order to maintain reliable operation without greatly increasing power device costs, RF induction power supplies must appropriately respond to varying loads. To maintain high efficiencies, Induction power supplies typically resonate a capacitor or heating coil in combination with the part to be heated. As the part is heated, the natural resonant frequency of the system is affected. To be cost effective and reliable, induction supplies must respond by sensing and adjusting (varying) the operating frequency to maintain resonant operation.
To maintain a constant output power, the specific amount of time that switching devices are allowed to conduct (i.e., the “on-time”) must be corrected for as resonant frequency (fr) changes. More specifically, to maintain a constant power output, the switching devices must be allocated an on-time that is proportional to the cycle time (cycle time=1/fr).
This can be done in a closed loop control circuit by sensing the output voltage and adjusting the switching devices on-time. Doing this affects resonant frequency sensing so that two loops are interactive and typically require compensation to slow their individual response. Another way to accomplish proportional on-time is through prediction, allowing the two loops to run independently, providing a much faster response to changing loads.